Graham Shields writes:

As Scotland’s west coast recedes from view, the ocean resembles a mirror, broken only by the swash of the boat and the dolphins chasing us. We’re headed to the craggy, uninhabited islands known as the Garvellachs. Only reachable during Scotland’s short summer, there is nothing between here and North America, and so landing – or rather jumping hopefully onto slippery rocks – is dependent on the kindness of the Atlantic swell. We’ve come to see a globally unique suite of rocks, which preserve a precise moment of global significance: when a warm, tropical environment, teeming with photosynthetic life-forms such as cyanobacteria and algae, transitioned abruptly into perpetual cold.

Imagine a planet suffocated by ice, its glaciers stretching unbroken from the poles to the equator. Such an event, if it transpired on Earth today, would see kilometres-thick ice sheets gouging their way from the Arctic to the Bahamas. Once-diverse ecosystems and climate zones would merge into a single, uniform condition, seemingly destined to be barren. Scientists once argued that such a ‘snowball’ state could never have existed on Earth since global glaciation could not be reversed. Moreover, on such a world, all life, including our own ancestors, would surely have been extinguished. However, hard evidence can change even the most fixed mind, and what was once inconceivable is now the prevailing wisdom. Today’s scientists agree that ice did indeed reach the equator on at least two occasions between 717 and 635 million years ago, where it stayed for tens of millions of years.

What’s more, I and many of my colleagues now think that life not only survived this frozen age, but that such extreme conditions may even have helped life become more complex, a process that would eventually lead to the evolution of all animal forms. Quite simply, without Snowball Earth, you and I wouldn’t be here. And embedded in the rocks of these remote Scottish islands, you can see how it happened. [Continue reading…]